“RAFT” is an acronym that stands for reversible addition-fragmentation chain transfer. It is but one of several controlled free radical polymerization (CFRP) systems in general use. RAFT is a robust and versatile system and amenable to emulsion polymerization. The RAFT process is mediated by RAFT agents. These agents are typically derived from specific dithioesters, xanthates, dithiocarbamates and trithiocarbonates designed to have both high chain transfer constants and good radical polymerization reinitiating efficiency.
With the current state of RAFT-CFRP technology, polymers of low polydispersity (PDI) and complex architecture can be synthesized in bulk or solution with relative ease. The major obstacle that remains to their widespread commercial use has been the difficulty in adapting CFRP methods to polymerization in aqueous dispersion (emulsion). Polymerization in waterborne organic dispersions is an environmentally superior way to conduct radical polymerizations because the need for organic solvents is eliminated.
The degenerative RAFT mechanism does not drastically affect the radical concentration in the particle which in turn, leads to a similar kinetic behavior seen in typical emulsion polymerization. Unfortunately, the use RAFT agents in typical ab initio (unseeded) emulsion polymerization have not proven to be straightforward. Problems include severe retardation, loss of molecular weight control, loss of colloidal stability and/or the formation of highly colored oily layers. Various reasons for this behavior are possible, such as the formation of destabilizing oily oligomers, and poor transport of RAFT agent from the monomer droplets through the water phase to the loci of polymerization in the micelles.
To overcome these problems and achieve successful emulsion polymerization under RAFT control, several techniques have been developed. One method uses the combination of preformed seed latex, a highly hydrophobic RAFT agent and acetone as an additive to aid in the transport of the RAFT agent through the water phase and into the seed particle. Subsequent swelling of the seed particles with monomer in the presence of a water soluble initiator lead to a controlled free radical polymerization. Another method uses an amphipathic RAFT agent which can mediate polymerization in both the aqueous and organic phases. RAFT in miniemulsion systems has also been tried with mixed results. While many of these techniques have been shown to work, most lack generality or suffer practical difficulties for commercial application.